Various multiple access methods based on the orthogonal frequency division multiplexing (OFDM) method have been studied as candidate methods for satisfying users who require a high-speed multimedia service for next generation mobile communication. Among those methods, an orthogonal frequency: division multiplexing access (OFDMA) method, based on the OFDM method, is for achieving multiple accesses by forming respective user data from subsets among a total of orthogonal sub-carriers. In the OFDMA, the respective subsets of the subcarriers are allocated to each mobile station, and allocated units (i.e., subsets) are called subchannels.
In an uplink of a mobile communication system, capacity for the mobile communication system is increased by minimizing interference between mobile stations, and transmit power is controlled such that a base station may receive signals from the mobile stations with constant strength for the purpose of improving communication quality.
Since received signal strength (RSS) is in inverse proportion to a distance between the base station and the mobile station in a conventional code-division multiple access (CDMA) mobile communication system or IEEE 802.11 orthogonal frequency division multiplexing (OFDM) method, signals transmitted from the mobile station may be received in the base station with constant strength by increasing the transmit power for a mobile station far from the base station and reducing the same for a mobile station close to the base station.
Methods for controlling the transmit power in the conventional OFDMA system have been disclosed in Korean Patent Publication No. 2002-0081154 “Method for control power of forward communication channel on OFDMA mobile communication system”, and in the method, reliability of a communication channel may also be maximized by adaptively controlling the power of the forward communication channel according to the distance between the base station and the mobile station or varied locations of the base station and the mobile station.
In addition, amplitude of an output signal of a modulator varies according to the number of subchannels since the number of subchannels varies according to a communication environment in the OFDMA system. Accordingly, the transmit power of the mobile station may vary in the same place according to variations of the number of subchannels.
Accordingly, the transmit power of the mobile station for the uplink transmission in the OFDMA system is required to be controlled by synthetically considering the distance between the base station and the mobile station, the number of subchannels, and modulation levels. In addition, the transmit power is required to be controlled such that the signals transmitted from the mobile stations may be received in the base station with the constant-strength while a signal to noise ratio (SNR) of the transmitted signal is maintained at a predetermined level regardless of the variation of the number of subchannels.
However, in the conventional CDMA mobile communication system or IEEE 802.11 OFDM method, the amplitude of the output signal of the modulator is constant since the number of subchannels does not vary according to the communication environment. Accordingly, it has not been considered to control the transmit power according to the number of subchannels used in the OFDMA system since the SNR and the transmit output according to the variation of the number of subchannels are not reduced when controlling the uplink power.
Even in the OFDMA system, a power control algorithm has been relatively simple since the uplink transmit power control has conventionally been performed by considering the received signal strength according to the distance between the base station and the mobile station. However, the SNR and the transmit output are reduced according to the variation of the number of subchannels used in the system, and therefore an operational area of a transmit output end is required to be widened to be capable of processing the reduction.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.